1. Field of the Invention
The present invention relates to a directional resonance magnetometer. The latter is more particularly intended for the measurement of weak magnetic fields, e.g. approximately 0 to 100 microteslas.
Thus, particular applications of the invention are in geophysics, mining prospecting and space detection.
The magnetometer according to the invention is directional, i.e. it is associated with one measurement direction and the magnetometer makes it possible to determine the value of the projection of a magnetic field, which is a vector quantity, on the measurement direction.
The pass band of such a magnetometer is between 0 and 10 Hz and can even extend to a few kHz.
The expression of the resolution of the magnetometer must take account several parameters:
a) the spectral density of the noise brought to the input, the density being a function of the measurement frequency f in the general case and expressed in n/T.Hz.sup.1/2. PA1 b) the existence of a noise in f.sup.-a, where a is an integer, which generally increases at low frequencies, PA1 c) the drift during time and as a function of temperature and PA1 d) the stability of the zero with the presence or absence of a false zero. PA1 Mo=4 pi.10.sup.-7 H/m, PA1 g: gyromagnetic ratio of the electron (g/2pi=28 GHz/T), PA1 Ho: magnetic field applied to the spins at resonance (expressed in A/m), PA1 pi: well-known symbol representing approximately 3.1416. PA1 a sample made from a material having resonant spins and first magnetic means placed in the vicinity of the sample and which excite the resonance of the spins and for detecting the excited resonance, PA1 processing means connected to the first magnetic means and to the second magnetic means and making it possible to obtain on the one hand voltage values respectively proportional to (Hb-Ho+Hlex) and to (Ho-Hb+Hlex) in the vicinity of resonance, Ho being the value of the magnetic field applied to the sample at resonance and being well above Hex, (i.e. at least equal to approximately 10 time Hex), and on the other hand a pulsed square-wave current making it possible to induce the field Hb via second magnetic means, and PA1 closed loop control means connected on the one hand to the processing means and on the other to the second magnetic means for compensating the field Hlex and which supply a voltage proportional to the field Hlex. PA1 a sample and hold circuit, whose input is connected to the output of the first electronic device and whose output controls the closed loop control means, PA1 a voltage source, PA1 a second electronic device supplied by the source and permitting a voltage gain of alternatively +1 and -1, and PA1 another voltage-current converter, whose input is connected to the output of the second electronic device and which supplies the second magnetic means.
2. Discussion of the Background
Directional magnetometers of the flux gate type are already known and a good description thereof is provided in a document (1) by F. M. Neubauer et al, The Giotto Magnetometer Experiment, J. Phys. E:Sci. Instrum., 20, 1987, pp. 714 to 720 which, like the other documents cited hereinafter, is mentioned at the end of the present description.
The noise limit of such flux gate magnetometers is approximately 10 pT. Zero stability effects exist in such magnetometers, which have very high performance characteristics, but are expensive. If such a magnetometer is exposed to high magnetic fields, impacts or pressures, its characteristics can be very detrimentally affected.
In addition, U.S. Pat. No. 3,191,118 and the reference to D. Bourdel et al., An Electronic Resonance Magnetometer, Performance Optimization, Revue de Physique Appliquee, Vol. 5, February 1970, pp. 188 to 190, and D. Bourdel et al., Improvement To The Sensitivity Of Paramagnetic Resonance Magnetometers, Revue de Physique Apliquee, Vol. 7, March 1972, pp. 23 to 27, disclose directional resonance magnetometers.
Known directional flux gate magnetometers and directional resonance magnetometers suffer from the disadvantages referred to hereinafter.
Flux gate magnetometers suffer from drifts in time and as a function of the temperature. They are subject to a false zero phenomenon (i.e. they give a response which is not necessarily zero in the presence of a zero magnetic field) and are irreversibly sensitive to high magnetic fields, shocks and pressures.
The known directional resonance magnetometers have large overall dimensions (their volume possibly extending to several dm.sup.3), while having a low resolution of approximately 1 nT.
The present invention proposes a directional resonance magnetometer, whose performance characteristics are identical to those of the flux gate type, while having limited thermal and time drifts, which is not subject to the false zero phenomenon and whose sensitivity to high magnetic fields, shocks, impacts and pressure is reversible. Moreover, the size of the magnetometer according to the present invention can be reduced to a few millimeters.
Reference will subsequently be made in connection with FIGS. 1 to 6 to various devices known from the magnetometry and spectrometry fields.
FIG. 1 diagrammatically shows a magnetometer known from the above-cited document D. Bourdel et al., An Electronic Resonance Magnetometer, Performance Optimization, Revue de Physique Appliquee, Vol. 5, February 1970, pp. 187 to 190 (see FIG. 4 therein). In this magnetometer, a sample 2 made from a material having resonant electron spins is exposed to a magnetic polarization field Hb and an external magnetic field to be measured Hex, the latter being e.g. the earth's magnetic field. The polarization field Hb is created by any appropriate means, e.g. a not shown magnetic coil placed in the vicinity of the sample and which is supplied by a current having a variable intensity.
The magnetometer of FIG. 1 also has an exciting and receiving coil 4 surrounding the sample 2. The axis of the coil 4 is perpendicular to the direction D of the polarization field Hb, which is the measurement direction of the magnetometer of FIG. 1.
In the case where the electron spins resonate, the resonant frequency fo of these electron spins is given by the Larmor relation: EQU fo=g.Mo.Ho/(2pi)
with
A high frequency generator 6 set to fo excites the resonance via a measuring bridge 8 and the coil 4, which forms part of a resonant circuit 10. The latter is tuned to the frequency fo and comprises, apart from the coil 4, a variable capacitor c1 connected between the terminals of the coil 4. The resonance is detected by means of the coil 4, the measuring bridge 8 and a receiver 12, which has an amplitude detection. The measuring bridge 8 serves to decouple the reception from the excitation.
It is pointed out that the measuring bridge 8 can be eliminated, provided that there is a coil emitting both orthogonally to the direction D and to the axis of the coil 4, connected to the generator 6, the resonant circuit 10 in this case being only connected to the receiver 12.
An observation device 14 receives at its input a detected voltage V supplied by the output of the receiver 12. The device 14 supplies a curve, which is shown in FIG. 2 and which represents the variations of the voltage V as a function of the magnetic field H applied to the electron spins. It is an absorption curve with a gaussian shape. Resonance takes place when H is equal to Ho. The field applied H is the vector sum of the polarization field Hb and the external field to be measured Hex.
A field Ho much higher than the field Hex is chosen, so that H can approximately be expressed by the following formula: EQU H=Hb+(Hex.cosTh)
in which Th is the angle between the vectors, which represent Hb and Hex, which confirms the directional character of the magnetometer. It should be noted that this formula makes it possible to determine Hex, knowing Th, Ho and the value Hbo of Hb, which leads to Ho.
Apart from obtaining this simple formula another interest of having Ho very high is that the signal-to-noise ratio (s/n ratio) of the magnetometer is, at a first approximation, proportional to fo.sup.2 and therefore Ho.sup.2.
FIGS. 1 to 3 of the document to D. Bourdel et al., An Electronic Resonance Magnetometer, Performance Optimization, Revue de Physique Appliquee, Vol. 5, February 1970, pp. 187 to 190 show constructional variants of the magnetometer diagrammatically shown in FIG. 1 of the present description.
A device disclosed in French Patent Application 8809830 and EP-A-0359598 from the spectrometry field, but which is not used in the magnetometry field is diagrammatically shown in FIG. 3 and reveals a coherent detection of the high frequency.
The device of FIG. 3 uses the sample 2, the field Hb, the coil 4, the generator 6, the measuring bridge 8 and the resonant circuit 10 of FIG. 1, arranged in the same way and also has a low noise amplifier 16, a balanced mixer 18 and a low-pass filter 20.
The high frequency signal which is available at the output of the measuring bridge 8 is amplified by the amplifier 16 and is supplied to the input of the balanced mixer 18, whose reference signal is an "image" of the high frequency excitation.
More precisely, this reference signal is a signal having the same frequency as that of the excitation signal supplied by the generator 6 to the measuring bridge 8, but whose amplitude and phase can be made different from those of the latter signal.
The output of the mixer 18 is filtered by the low-pass filter 20 in order to eliminate the residues, as well as the harmonics of the high frequency.
By connecting to the output of the low-pass filter 20 an appropriate, but not shown observation device, it is possible to obtain, as a function of the respective phases of the exciting signal and the reference signal, the curve of FIG. 2 or the curve of FIG. 4, which represents the variations of a voltage V1 as a function of the field H and which is called the dispersion curve. This curve of FIG. 4 can be used as it is for a magnetometer, with linearity and drift limits, or as a zero signal in a magnetic field close loop control device.
Another device known in the field of spectrometry and used in the magnetometry field is diagrammatically shown in FIG. 5 and makes it possible to obtain the derivative of the absorption curve. The device of FIG. 5 uses the sample 2, the field Hb, the coil 4, the generator 6, the measuring bridge 8, the resonant circuit 10, the amplifier 16 and the balanced mixer 18 of FIG. 3, arranged in the same way.
The device of FIG. 5 also has an oscillator 22, which produces a signal having an audio frequency fm, as well as a coil 24, which receives the said signal and which produces in the sample 2 a magnetic field of frequency fm, called the "agitation field" and which is superimposed on the field Hb, the axis of the coil 24 coinciding with the axis of the not shown coil used for producing the field Hb.
At the output of the balanced mixer 18, the low-pass filter 20 of FIG. 3 is replaced by a band-pass filter 26 around the frequency fm.
A phase shifter 28 receives the high frequency signal from the generator 6 and supplies the balanced mixer 18 with a signal having an adequate phase for obtaining, at the output of the mixer 18, the voltage V of the curve shown in FIG. 2.
A synchronous detection means 30 at the frequency fm and which is controlled by a reference signal from the oscillator 22, has its input connected to the output of the filter 26 and supplies at the output a voltage Vs. The reference signal has a frequency fm, but its amplitude and phase can be made different from those of the signal supplied by the oscillator 22 to the coil 24.
By connecting to the output of the synchronous detection means 30 an appropriate, but not shown observation means, it is possible to observe a curve which is shown in FIG. 6 and has no offset voltage. This curve represents the variations of the voltage Vs as a function of the field H and constitutes the derivative of the curve of FIG. 2.
The parameters making it possible to optimize the resonant materials and the respective amplitudes of the high frequency field and the agitation field are obtained by a process described in French Patent Application 8809830 and EP-A-0359598.
Thus, it is known to produce a directional magnetometer with the aid of the following means: a sample formed from an appropriate resonant material (see e.g. French Patent Application 8612278) one or more sampling and exciting coils, a device making it possible to produce a continuous polarization field Hb (close to Ho), an oscillator at resonance and an electronic means making it possible to obtain the curve of FIGS. 4 or 6.
In this case, a variation of the external field Hex, to the extent that it is well below the line width DH1 (curve of FIG. 4) or DH2 (curve of FIG. 6) leads to a resonance variation and to an image voltage at the output of the magnetometer.
The linearity can be improved by a field feedback using the voltage V1 (FIG. 4) or the voltage Vs (FIG. 6) as a zero indicator, by integrating the voltage and by reinjecting a current proportional thereto and which creates a magnetic field as a result of a coil, whose axis is parallel to the direction of Hb. Such a procedure is standard in magnetometry (see, e.g. French Patent Application 8717566).
The main disadvantage of such a magnetometer is linked with the stability of the polarization field Hb. The requisite precision can be evaluated with the aid of a following example.
Consideration is given to a field to be measured Hex of 50 microteslas with a precision of 10 pT and a polarization Hb 100 times higher than Hex, i.e. a field Hb of 5 mT. Any variation of Hb will be interpreted as a variation of Hex. Consequently the field Hb must be stable to within 10 pT, i.e. 2.10.sup.9 in relative precision. This is virtually impossible to achieve with known procedures.
One solution which has been tested consists of pulsating the field Hb at a frequency fp well below fo, so that the field Hb alternately describes the resonance line in a positive and negative manner see U.S. Pat. No. 3,191,118 and the references to D. Bourdel et al, An Electronic Resonance Magnetometer, Performance Optimization, Revue de Physique Appliquee, Vol. 5, February 1970, pp. 188 to 190, and D. Bourdel et al., Improvement To The Sensitivity Of Paramagnetic Resonance Magnetometers, Revue de Physique Apliquee, Vol. 7, March 1972, pp. 23, to 27. Thus, the stability problem to be solved is brought down to obtaining a short term stability over a period 1/fp.
The problem associated with the above solution is that the resonance is not exploited in its maximum sensitivity zone (see French Patent Application 8809830 and EP-A-0359598) except during a minute fraction of the scan time 1/fp, which is of the sinusoidal or triangular type.